1. Field of the Invention
The invention relates to solvent-containing single-component polyurethane (PUR) baking enamels comprised of polyisocyanates partially blocked with blocking agents and hydroxyl-group-containing polyesters based primarily on aromatic dicarboxylic acids, diols, and triols, and further possibly containing customary additives.
2. Description of the Background
In order to prepare mixtures of polyisocyanate and polyhydroxy compounds, which mixtures are maintainable at room temperature, one employs polyisocyanates, the reactive groups of which are blocked with monofunctional compounds. The polyisocyanates are generally referred to as "capped" or "blocked" polyisocyanates. Such products are described, e.g., in 1949 Annalen, Vol. 562, pp. 205-229.
Under the influence of relatively high temperatures, the blocking groups cleave thereby liberating the starting polyisocyanate compounds. The newly unblocked isocyanate groups can then react with the hydroxyl groups present in the reaction mixture, with the formation of a product film.
If these mixtures of blocked polyisocyanates and polyhydroxy compounds are to be employed for coating objects in injection molding and casting processes, or for coating metal plates by roll-coating, particularly in "coil coating" processes, the crosslinking agent as well as the resin must be soluble in the usual enamel solvents and must be storage-stabile.
With few exceptions, aliphatic polyisocyanates blocked with epsilon-caprolactum and/or ketoximes are incompatible with the usual enamel solvents (Ger. AS 23 46 818, col. 2, Line 18). Therefore, the aliphatic polyisocyanates are not acceptable starting components for the preparation of single-component PUR baking enamels.
German AS 23 46 818, in col. 2, line 45 describes that it is advantageous to leave free isocyanate groups in the amount of 0.01-5 wt.% in the blocked isocyanate adduct. In German OS 33 22 722 up to 10 wt.%, preferably up to 5 wt.%, of free isocyanate groups remain in the blocked isocyanate adduct. This upper limit of free isocyanate groups in a blocked isocyanate adduct has not heretofore been exceeded, because of the need to control viscosity and to avoid the tendency to gelify, i.e. on grounds of ensuring the processibility of the single-component PUR baking enamels. The maximum possible content of free isocyanate groups is favored, for a number of reasons, including ecological, economic, and storage stability reasons. The binder components have excellent compatibility in enamel solvents.
A need therefore continues to exist for a single-component PUR baking enamel which is prepared from a partially blocked polyisocyanate in which the content of free isocyanate groups is greater than that which has been customary for partially blocked polyisocyanates.